White LEDs that combine a LED element that emits light having a short wavelength such as a blue light or an ultraviolet light and a fluorescent material that is excited by absorbing some or all of the light emitted from the LED element and then emits a fluorescent light of a longer wavelength such as a yellow fluorescent light are already known. For example, white LEDs composed of a compound semiconductor blue LED element and a cerium-activated yttrium-aluminum-garnet fluorescent material that absorbs blue light and then emits a yellow fluorescent light that is a complementary color to blue are disclosed in Japanese Patent (Granted) Publication No. 2,900,928, Japanese Patent (Granted) Publication No. 2,927,279, and K. Bando, K. Sakano, Y. Noguchi and Y. Shimizu, “Development of High-bright and Pure-white LED Lamps,” J. Light & Vis. Env. Vol. 22, No. 1 (1998), pp. 2 to 5.
Adding a red fluorescent material to compensate for the lack of a red component is also a known technique, and Japanese Unexamined Patent Application, First Publication No. 2003-321675 discloses a technique in which a red fluorescent material such as (Sr1−x−y−zBaxCay)2Si5N8:Euz2+, SrS:Eu, CaS:Eu or (CaxSr1−x)S:Eu2+ is added to a white LED composed of a blue LED element and a cerium-activated yttrium-aluminum-garnet fluorescent material. Similar techniques are also disclosed in Japanese Unexamined Patent Application, First Publication No. 2003-321675 and M. Yamada, T. Naitou, K. Izuno, H. Tamaki, Y. Murazaki, M. Kameshima and T. Mukai, “Red-Enhanced White-Light-Emitting Diode Using a New Red Phosphor” Jpn. J. Appl. Phys. Vol. 42 (2003) pp. L20 to L23.
As another example of a known technique that uses a blue LED element, Japanese Unexamined Patent Application, First Publication No. Hei 10-163535 discloses a technique for realizing a white LED using a blue LED element, a fluorescent material that is excited by blue light and emits a green light, and a fluorescent material that is excited by blue light and emits a red light. Furthermore, a white LED that uses SrGa2S4:Eu2+, which is a fluorescent material that is excited by blue light and emits a green light, and SrS:Eu2+, which is a fluorescent material that is excited by blue light and emits a red light, is disclosed in Paul S. Martin, “Performance, Thermal, Cost & Reliability challenges for Solid State Lighting,” OIDA Conference, May 30, 2002.
Moreover, as an example of a known technique that uses an ultraviolet LED element, Published Japanese Translation No. 2000-509912 of PCT discloses a technique for realizing a white light having a high color rendering characteristic by using a combination of an ultraviolet LED element, a fluorescent material that is excited by ultraviolet light and emits a blue light, a fluorescent material that is excited by ultraviolet light and emits a green light, and a fluorescent material that is excited by ultraviolet light and emits a red light.
On the other hand, recently, there has been extensive research on oxynitride fluorescent materials and nitride fluorescent materials, and for example, Japanese Unexamined Patent Application, First Publication No. 2002-363554 discloses a europium (Eu)-activated calcium (Ca) solid solution α-SiAlON fluorescent material. This fluorescent material is excited by blue light and emits a yellow light, and can therefore be used favorably as a wavelength conversion material for a white LED. This fluorescent material is described in detail in R. J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto, M. Mitomo, “Eu2+-doped Ca-α SiAlON: A yellow phosphor for white light-emitting diodes,” Appl. Phys. Lett., Vol. 84, pp. 5404 to 5406 (2004). A low color temperature white LED lamp that uses this fluorescent material and exhibits excellent chromaticity stability upon temperature variation is disclosed in K. Sakuma, K. Omichi, N. Kimura, M. Ohashi, D. Tanaka, N. Hirosaki, Y. Yamamoto, R.-J. Xie, T. Suchiro, “Warm-white light-emitting diode with yellowish orange SiAlON ceramic phosphor,” Opt. Lett. Vol. 29, pp. 2001 to 2003 (2004).
Conventionally, fluorescent materials have been predominantly oxides or sulfides, and have required further improvement in terms of their durability and their properties under high-temperature environments. In recent years, development efforts have been focused on developing oxynitride fluorescent materials and nitride fluorescent materials that exhibit superior long-term reliability and high-temperature properties. Moreover, although extensive research is also being conducted into solid-state illumination devices that use a combination of a semiconductor light emitting element and a fluorescent material, oxynitride fluorescent materials or sulfide fluorescent materials are the primary materials being researched. However, even if these conventional fluorescent materials are combined appropriately, and a white LED is then fabricated using a combination of these fluorescent materials and a blue LED element, achieving a white LED having a high color rendering characteristic has proven difficult.
On the other hand, although a white LED using an ultraviolet LED element such as that disclosed in Published Japanese Translation No. 2000-509912 of PCT is capable of realizing a superior color rendering characteristic, a significant problem arises in that the ultraviolet light from the light emitting diode element tends to cause degradation of the sealing resin and the package. This is a major problem in terms of the long-term reliability of the device.
Moreover, other problems include possible external leakage of ultraviolet light that has not undergone wavelength conversion, and the fact that the light emission efficiency of an ultraviolet LED element is lower than that of a blue LED element.